White crust on the inside of a Black Birch bark.
Berlin MA, 11/27/08

The reason this is interesting, to me, is that I found a similar white mold growing on the first Inonotus obliquus fruiting body suspect.  Eventually it spread all over the 3 fruiting bodies on that Yellow Birch.  In this case it is not a mold but a crust.  Here is the inside of bark piece I tore off.

Close-up

It left a thick coating of white spores overnight.  The larger spore measured 8.6 x 6.7 microns.